This invention relates to an antenna and, more especially, this invention relates to an antenna enabling the adaptive control of beam shape and directivity of the antenna.
Known conventional low-cost directive antennas typically comprise so-called end-fire arrays or dish or horn designs in order to obtain required directivity and beam shape. Angular direction is determined by mechanical orientation of the antenna. Beam shape is determined by the physical size and geometric form of the dish or horn. Other known directive antennas may be phased array antennas. A phased array antenna comprises a plurality of transmit or receive elements, each of which is essentially non-directive but whose co-operative effect may be a highly directive and steerable beam. Phased array antennas tend to be large, costly and complex.
It is well known that electromagnetic radiation may be directed and otherwise controlled through reflection from conducting surfaces. Examples of reflective control would include array antennas and aerials, and dishes such as are used in microwave receivers and transponders. Although normally associated with metallic high-conducting surfaces or elements, it has been shown that semi-conducting materials may also be used to reflect or otherwise modify electromagnetic radiation. Furthermore the degree of conductivity of a semi-conductor may be readily modified by the influence of incident illumination by light or the electrical injection of carriers, (T. S. Moss, xe2x80x9cOptical Properties of Semiconductorsxe2x80x9d, Butterworths, London (1959)). The rate of change of conductivity (recombination rate) and the amount of energy required to sustain the process is determined by the free carrier lifetime, which may be greatly influenced by known surface passivation techniques that serve to reduce crystalline dislocations and impurities within the semiconductor where free carriers can recombine. Typical semiconductors in widespread commercial use include, for example, Si, GaAs, InGaAsP, InP.
Intrinsic semiconductor materials may be doped with impurities to produce materials with precisely controlled conductivity. Light of sufficiently short wavelength, as may be determined by the bandgap Ev characteristic of the semiconductor material, may be used to increase the density of free carriers in said semiconductors. Prior art shows that the intensity of an optical illumination changes the complex refractive index of semiconductors. The mechanism of this phenomenon is described by fundamental Drude theory, (see for example I Shih, xe2x80x9cPhoto-Induced. Complex Permittivity measurements of Semiconductorsxe2x80x9d, 477 SPIE 94 (1984), and B Bennett, xe2x80x9cCarrier Induced Change in Refractive Index of InP, GaAs, and InGaAsPxe2x80x9d, 26 IEEE J. Quan. Elec. 113 (1990).
Lev S. Sadovnik, et al (U.S. Pat. No. 5,305,123, LIGHT CONTROLLED SPATIAL AND ANGULAR ELECTROMAGNETIC WAVE MODULATOR, and U.S. Pat. No. 5,982,334, ANTENNA WITH PLASMA-GRATING) illuminated the surface of a semiconductor waveguide to produce adaptive diffraction gratings for angular and spatial control of electromagnetic radiation, and also used locally induced plasma to produce optically controlled switches (U.S. Pat. No. 5,796,881, LIGHTWEIGHT ANTENNA AND METHOD FOR THE UTILIZATION THEREOF). The same researchers used PIN semiconductor structures to inject carriers into an intrinsic semiconductor to create a pattern of localised regions of high carrier density and thereby form a diffraction grating.
It is an aim of the present invention to provide an antenna which can be manufactured at low cost and which can be used in a wide variety of applications.
Accordingly, in one non-limiting embodiment of the present invention, there is provided an antenna comprising:
(a) semi-conductor means having upper and lower surfaces, the upper and lower surfaces having a pattern of electrically conducting regions;
(b) first generating means for generating conducting plasma filaments of charged carriers between the upper and the lower conducting regions;
(c) radio frequency feed means to selected ones of the conducting plasma filaments in order to couple radio frequency energy to or from the semi-conductor means; and
(d) second generating means for selectively generating a pattern of conductive filaments between the surfaces of the semi-conductor means in order to reflect and thereby to focus an electromagnetic wavefront incident upon an edge of the semi-conductor means to at least one radio frequency feed point within the semi-conductor means;
and the antenna being a planar dielectric lens antenna with controlled conductive elements forming a directive antenna for the reception or transmission of a beam of radio frequency energy in the plane of the semi-conductor means.
The antenna of the present invention may be a low cost adaptive antenna which is able to be used in a wide range of applications including, for example, telecommunications, radar, and tracking of base stations from vehicles to satellite or other such mobile links. The antenna of the present invention may be a broad-band width antenna with multi-beam directivity control. The antenna of the present invention may encompass relatively long centimetric radio-frequency wavelengths, through millimetric wavelengths to long optical wavelengths such as infrared wavelengths.
The first generating means is used to increase locally the carrier density within a semiconductor volume to produce the conducting plasma filaments. The conducting filamentary plasma is well confined to the volume between the surface regions of high conductivity, and it extinguishes rapidly in the absence of the first generating means. The locally defined conducting plasma filaments may be used firstly to reflect or absorb incidence electromagnetic radiation according to their carrier concentration within a wave-guiding structure such for example as a planar circular semi-conductor lens providing 360xc2x0 coverage of controllable beam width and side lobe level. The locally defined conducting plasma filaments may be used secondly to provide an antenna feed means analogous to an electrical dipole or similar radio frequency feed within the wave guide structure.
The antenna may be one in which the regular matrix of filaments is in the form of a plurality of concentric rings of points thereby to enable simulation of a quasi-planar reflector.
The antenna may be one in which the first generating means is electrical bias means for providing an electrical bias potential between the said electrodes on the upper and lower surfaces. The semi-conductor medium may advantageously comprise a plurality of regions of differential impurity doping thereby to enhance carrier generation.
Alternatively, the antenna may be one in which the first generating means is optical projection system first generating means, and in which the antenna is controlled by selective illumination of the semi-conductor means through the optical projection system first generating means.
The optical projection system first generating means may comprise a plurality of the optical fibres which couple light to the surface of a layer of the semi-conductor means, the optical fibres being arranged so as to provide a plurality of light injection points in the form of a selectable array.
Usually, the antenna will be a flat circular dielectric lens antenna. Also usually, the semi-conductor means will be a semi-conductor plate. The semi-conductor plate may comprise selectively doped regions. Preferably the semi-conductor plate is a disc but other shapes for the semi-conductor plate may be employed if desired.
The antenna may include a shaped dielectric medium concentric with the perimeter of the semi-conductor means, whereby electromagnetic coupling between the antenna and an external medium is enhanced.
The antenna may be one in which the pattern of conducting plasma filaments is configured so as to focus electromagnetic energy from an external medium to a point feed within the semi-conductor means, a radio frequency feed at the focal point enabling electromagnetic coupling to or from the antenna.
The apparatus may be one in which the conducting plasma filaments are configured in patterns of sub-arrays such as to modify the beam shape and efficiency of the antenna. In this case, the conducting plasma filaments may be configured to produce multiple antenna beams.
The antenna may be one in which the conducting plasma filaments have a density which is controlled so as to enable reflected amplitude weighting within an array of elements.
The antenna may include a toroidal dielectric annulus in proximity with the perimeter of the semiconductor means, whereby electromagnetic coupling between the antenna and an external medium is enhanced.
The antenna may form part of a plurality of the antennas, the antennas being mounted in an array to enable elevation control of the resultant beam in conjunction with azimuthal control. In this case, the antennas are preferably mounted in a stack but other configurations may be employed if desired.
The antenna may be one in which the conducting plasma filaments are produced by other means, to include photo-conduction, current injection, ferro-electric and ferro-magnetic effects.
The antenna may be one in which the semi-conductor means comprises a semi-conducting dielectric medium of polycrystalline or amorphous form.
The antenna may be one in which the active medium is of photo-conductive or electro-conductive plastic.
The antenna may be one in which the beam of radio frequency energy which is controlled by the antenna is of wavelengths characteristic of electro-optics rather than microwave radio frequencies.
The antenna may be one which is designed by calculation of geometry and material properties to perform specific applications relating to telecommunications, radar, medical scanning, inspection or other forms of sub-surface imaging.
The antenna may be complemented to allow controlled reflection of an illuminating signal by varying the density of the filamentary plasma containing the plasma filaments, the antenna then functioning as a transponder capable of both directing and modulating a reflected signal.
At lower frequencies, where the diameter of the planar dielectric lens antenna approaches a half wavelength (in dielectric) and its thickness is very much less than half a wavelength (in dielectric), the active antenna begins to operate as a dielectrically-loaded steerage cavity-backed slot antenna. That is, upper and lower surfaces of the semi-conductor means form a waveguiding structure which can be further constrained by a conducting plasma wall to create a reconfigurable cavity. This reconfigurable cavity can be fed either by a metal feed or a plasma feed connected between the two major conducting surfaces of the semi-conductor lens. The semi-conductor means may be metallised. The position of such an unbalanced feed within the reconfigurable cavity will largely determine the feed""s matching characteristics. As the operating frequency increases, a wide range of reconfigurable cavities can usefully be formed to include a range of wide-band horn structures (for example Vivaldi) which may be further adjusted to become complex reflecting surfaces that can sustain selective electromagnetic modes.